A multiphase rotating electric machine is driven by controlling a current by pulse width modulation (PWM). For example, in a case where the multiphase rotating electric machine is a three-phase motor, a voltage reference signal related to voltages applied to three-phase windings, respectively, is compared with a PWM reference signal that is a carrier wave, such as a triangular wave, and a current that flows to the three-phase motor is controlled by switching on and off switching elements of an inverter.
In a case where the inverter is connected to a capacitor, when no current flows to the inverter, the capacitor is charged as a current flows into the capacitor from a power source. On the other hand, when a current flows to the inverter, the capacitor is discharged as a current flows to the inverter from the capacitor. In a case where PWM control is performed, a capacitor current is pulsed because charging and discharging are repeated in the capacitor during one cycle of PWM control. The capacitor current is pulsed, noise is generated or the capacitor generates heat. Additionally, fluctuation of voltage applied to the inverter may result in poor controllability of the inverter current. The pulsation of a current that flows into the capacitor is a ripple current.
Thus, for example, in JP 2001-197779A, a phase difference is imposed on switching timings of switching elements between two sets of bridge circuits, based on pre-stored map data, so that a waveform of a summed capacitor current approaches a smooth waveform to reduce the ripple current. Additionally, in JP 2007-306705A, in a case where two axes are connected in a PWM amplifier, a voltage command value for one axis is biased to Vcc/4 (Vcc is a power source voltage) while a voltage command value for the other axis is biased to −Vcc/4 to reduce the ripple current.
However, JP 2001-197779A requires a delay circuit for imposing a phase difference on the switching timings according to a modulation ratio and a power factor, to output the resulting switching timings. Additionally, it is necessary to detect currents in a plurality of systems at short intervals, and operation load is heavy.
Additionally, in JP 2007-306705A, for example, in a case where two inverter systems are present, a voltage command value is biased to be higher by ¼ of a power source voltage in one of the two inverter systems. If the voltage command value is biased higher (upwards), the time for which a higher-potential-side switching element provided at a higher potential is turned on becomes longer than the time for which a lower-potential-side switching element provided at a lower potential is turned on. Additionally, in the other inverter system, the voltage command value is biased to be lower by ¼ of the power source voltage. If the voltage command value is biased lower (downwards), the time for which the lower-potential-side switching element is turned on becomes longer than the time for which the higher-potential-side switching element is turned on. If the time for which the higher-potential-side switching element is turned on is different from the time for which the lower-potential-side switching element is turned on, the amount of currents to be applied may be different, which may result in a difference in heat loss. If the difference occurs in heat loss between the switching elements, a marginal thermal design or an asymmetrical heat dissipation design is required. Otherwise, it is necessary to use respectively different elements as the switching elements.